The present invention relates to the field of wind turbines, especially to wind turbines having an electrically excited synchronous generator, and more particular to the excitation voltage supply of such a synchronous generator. Furthermore, the present invention relates to the start-up process of such a wind turbine.
Synchronous electric generators have a rotor which is excited with direct current, typically via slip rings. An alternating voltage is generated in the stator windings by the rotating field of the rotor. In common designs, the rotor excitation voltage is taken from the utility grid and supplied to the rotor via a separate circuit. However, in cases of low grid voltage or even grid failure, i.e. zero grid voltage, the excitation voltage is no longer sufficient to ensure that the generator is still excited. Thus, it is not guaranteed that the converter making the grid connection can stay online. In particular, if the declining voltage of the rotor windings is used to feed the converter during low or zero grid voltage, a sufficient supply can only be maintained for a short time depending on the time constant of the generator.
Other known designs propose the use of an uninterruptible power supply (UPS) for buffering the excitation circuit of the rotor. Due to the UPS, a sufficient excitation voltage can be supplied to the rotor windings even during longer periods of low or zero grid voltage. However, the use of a UPS causes additional costs, increases the number of parts to be maintained and increases the weight of the turbine.